Neuronal circuits in the central nervous system rely on the release of chemical neurotransmitters from specialized connections called synapses for communication. The major excitatory neurotransmitter is the amino acid glutamate, and release of glutamate from a pre-synaptic terminal elicits a response through activation of several types of receptors. One of the sub-types of glutamate receptors, the N-methyl-D-aspartate (NMDA) receptor, plays a major role in neuronal communication and in the plasticity of synaptic responses that occurs under both physiological and pathophysiological conditions.
NMDA receptors are ligand-gated cation channels comprised of a tetrameric assembly of NR1, NR2 and NR3 sub-units (Paoletti and Neyton, 2007). They are unique amongst neurotransmitter receptors in that they require occupation of two separate recognition sites for activation. An acidic amino acid site where glutamate binds is located on the NR2 sub-units, and a neutral amino acid (or co-agonist) site is located on the NR1 sub-unit. The endogenous co-agonist for this site was originally thought to be glycine, but more recent evidence indicated that D-serine is also an endogenous co-agonist. In fact, in higher brain regions D-serine may be the dominant co-agonist. Occupation of the co-agonist site is essential for glutamate (or a glutamate analog) to activate the NMDA receptor, and in native assays the removal of glycine or D-serine by exogenously-applied degradative enzymes can reduce or abolish NMDA receptor-mediated responses. For example, in the rat hippocampal slice, application of the D-serine metabolizing enzyme, D-amino acid oxidase (D-AAO), completely prevents the induction of long-term potentiation (LTP) a form of synaptic plasticity whose initiation is dependent on NMDA receptor activation (Yang et al., 2003). This suggests that the dominant co-agonist in this case is D-serine, since glycine is not a substrate for D-AAO.
The mechanisms that regulate extracellular D-serine, and therefore govern how NMDA receptors are activated, are not well understood. In keeping with other neurotransmitters and neuromodulators, it is likely that transporters on the cell surface are involved in regulating synaptic levels of D-serine. Amino acid transporters usually prefer L-amino acids, however D-serine has been shown to be a substrate for certain transporters. These include the heterodimeric transporter asc-1 (SLC3A2/SLC7A10) which has micromolar affinity for D-serine, ASCT2 (SLC1A5), ATBO+ (SLC7A9) and PAT1-4 (SLC36A1-4). Based on the tissue and cellular localization, the primary candidates for transporters that regulate synaptic D-serine levels are asc-1 (neuronal) and ASCT2 (glial). The related transporter ASCT1 (SLC1A4) also has been localized to neurons and glia, however it has been reported that D-serine is not a substrate for ASCT1 (Shafqat et al., 1993). None of these transporters are selective for D-serine, and their substrates are typically small neutral amino acids such as serine, alanine, cysteine and threonine. They also are known to function as exchangers that can flux their substrates both into and out of cells. Consequently, it has been unclear if these transporters are responsible primarily for the net uptake or the net release of D-serine and other substrates. Considering that asc-1 has the highest known affinity for D-serine, it has been thought that this transporter is primarily responsible for removing D-serine from the extracellular space (Rutter et al., 2007). In support of this, the asc-1 knock-out mouse has a phenotype that includes increased excitability (Xie et al., 2005).
In the CNS, NMDA receptors are important mediators of glutamate-mediated neurotransmission and synaptic plasticity. NMDA receptors occur throughout the brain and spinal cord and are widely considered to be essential for neuronal physiology (Traynelis et al, 2010). Based on experiments using exogenously-applied D-AAO, D-serine has been shown to be an endogenous co-agonist involved in NMDA-receptor-mediated synaptic responses in the forebrain (Henneberger et al, 2010; Fossat et al, 2012). In many regions of the CNS, NMDA receptors mediate the phenomenon of long-term potentiation (LTP), an important form of synaptic plasticity. NMDA receptor-dependent LTP is viewed as a mechanism of synaptic strengthening that is fundamental to the establishment and maintenance of appropriate synaptic connections. In the hippocampus, for example, LTP has been studied as a synaptic substrate of learning and memory (Citri and Malenka, 2007).
Certain CNS disorders are associated with a deficit in NMDA receptor function. In schizophrenia, the NMDA receptor hypofunction hypothesis was formulated to explain the “negative symptoms” and reduced cognitive functions that occur in this mental disorder and which are poorly treated by traditional antipsychotic drugs centered on dopamine. NMDA antagonists such as PCP, ketamine and MK-801 reproduce schizophrenic symptoms in humans, supporting the idea that reduced NMDA receptor function occurs in this disease (Coyle, 2006). Consequently, therapeutic strategies to enhance NMDA receptor function are currently being investigated. One example are inhibitors of transporters for glycine that are responsible for maintaining synaptic glycine concentrations. Since glycine, like D-serine, is an endogenous co-agonist at the NMDA receptor, inhibition of glycine re-uptake will increase synaptic glycine concentrations and increase NMDA receptor function. Currently, glycine transport inhibitors are in late stage clinical studies for the treatment of schizophrenia (Field et al, 2011). D-serine itself has been examined in several small schizophrenia trials as an adjunctive therapy with antipsychotic drugs (Labrie and Roder, 2010), with positive results. Consequently, the D-serine transport modulators described here are expected to be of utility in the treatment of schizophrenia, and we provide preclinical support for this idea. In addition, compounds that have the ability to enhance LTP have been shown to improve cognition in human subjects (Lynch et al, 2011). We show here that D-serine transport modulation enhances LTP not only in the visual cortex, but also in hippocampus, a region of the brain associated with learning and memory. Consequently, D-serine transport modulators will be useful as cognitive enhancing agents and can be used to treat diseases such as Alzheimer's disease or any CNS disorder where cognitive abilities are impaired.